In the practice of digital color image processing, an original color image, such as a photographic negative, is sampled periodically in three colors (e.g. green, red and blue) to produce a digital representation of the origional color image. The digital color image is processed by applying digital image processing functions to improve such image qualities as sharpness, tone-scale, and color balance. The processed digital color image is then displayed on a display medium such as photographic film or paper.
FIG. 2 is a schematic diagram illustrating apparatus for digital image processing. Such apparatus includes an input device 1 for sampling the original color image in each of three colors, and analog-to-digital converters 2 for producing the digital color image in the three colors. Commonly employed input devices include drum and flat bed scanners, linear and area solid state image sensing arrays, and CRT and laser flying spot scanners, each being provided with appropriate color filters to produce the color separations.
Each digital color separation image is stored in a mass storage memory 3, such as a solid state memory frame-buffer, magnetic tape or disc storage device. A digital computer 4 applies the various image processing functions to the digital color image to produce the processed digital color image.
The digital computer 4 may comprise a main frame general purpose digital computer, or for higher speed operation, a digital computer specially configured for high speed digital processing of color images.
The processed digital color image is converted to sampled analog form by digital-to-analog converters 5 and is displayed on an output device 6 such as a drum or flat bed graphic arts scanner, or a CRT or laser flying spot scanner. The elements of the image reproduction apparatus communicate via a data and control bus 7.
As noted above, among the processing functions performed by the digital computer are the improvement of the tone-scale and color balance of the color image. In the article entitled "Tone Correction of Color Picture by Histogram Modification" by Yoichi Miyake, Nippon Shashin Sakhaishi, V. 48(2), pp. 94-101, 1980, the author proposes a digital color image processing method wherein the tone-scale corrections are effected by modifying the histogram of color values of the green separation image. Color corrections are implemented by solving a conventional set of color masking equations of the form: EQU R'=a.sub.11 R+a.sub.12 G=a.sub.13 B (1) EQU G'=a.sub.21 R+a.sub.22 G=a.sub.23 B (2) EQU B'=a.sub.31 R+a.sub.32 G=a.sub.33 B (3)
where the matrix of color correction coefficients a.sub.ij are determined primarily by the characteristics of the input and output media.
In the field of optical photographic printing, it is well known to adjust the coefficients a.sub.ij of the masking equations to correct for "problems" in the color balance of the original, such as a color negative film balanced for daylight exposure that was exposed under tungsten lighting conditions. The correction is usually based upon the hypothesis that the large area integral of the colors in any scene should add up to a shade near gray. Full correction, i.e. forcing the integrated colors in the processed image all the way to gray will correct for problems in the original image caused by such factors as film keeping (i.e. problems that manifest themselves after the film has been stored for a time under conditions causing degradation of the film), incorrect exposure, or an improper match between the illuminant and the film balance. On the other hand, for scenes that do not obey the assumption that all the colors add up to gray (commonly called color subject failure scenes), full correction introduces unwanted color shifts in the printed image. Often some compromise between zero and full correction is employed in optical printers based upon the average characteristics of an expected population of photographic negatives.
This approach results in a large percentage of acceptable prints, but has a shortcoming in that it may not produce an "optimum correction" for any given original. Furthermore, there exists a class of "problem" originals that simply cannot be corrected by adjusting the coefficients of the masking equation. This class includes those originals in which the sensitometric curves cross each other. This phenomenon is caused, for example, by severe abuse to the original such as exposure of undeveloped photographic film to formaldehyde.
The objects of the present invention are directed to providing a digital color image processing method for adjusting the tone-scale and color balance of the processed image that overcome the shortcomings noted above.